This set of Cell Biology Multiple Choice Questions & Answers (MCQs) focuses on “Structure and Function of Membrane Proteins”.
1. Which of the following is covalently attached to a lipid molecule in cell membrane?
a) Integral protein
b) Transmembrane protein
c) Peripheral protein
d) Lipid-anchored protein
Explanation: Lipid-anchored proteins are located outside the lipid bilayer on either the extracellular or cytoplasmic surface. They are covalently linked to lipid molecules in the cell membrane. Peripheral proteins are associated to the cell membrane surface by non-covalent bonds.
2. Integral proteins are also known as ____________
a) Intrinsic proteins
b) Glycosylated proteins
c) Transmembrane proteins
d) Bilayer proteins
Explanation: Integral proteins are also known as transmembrane proteins as they pass entirely through the lipid bilayer. Transmembrane proteins have domains that protrude both from extracellular side and cytoplasmic side of the cell membrane.
3. The concept of transmembrane proteins was obtained from the results of which technique?
a) Freeze-fraction replication
b) Freeze-fracture replication
c) Fraction replication
d) None of the mentioned
Explanation: The concept that transmembrane proteins penetrate through the membrane was obtained from the results of freeze-fracture replication. In this procedure, a frozen solid tissue is fractured with a knife blade and metals are deposited on their exposed surfaces to form shadowed replica. This is viewed under electron microscope and it looks like a path strewn with pebbles, called as membrane-associated particles.
4. Transmembrane domain of a transmembrane protein is present as _______________
c) Triple beta-sheet
d) None of the mentioned
Explanation: The transmembrane domain of a cell membrane protein is the part that spans through the lipid bilayer of cell membrane. It mainly consists of a chain of about 20 non-polar amino acids arranged in an alpha- sheet structure.
5. Hydropathy plot helps to _______________
a) Identify hydrophobic amino acids
b) Determine arrangement of amino acids in a transmembrane protein
c) Provides an average hydrophobicity of short peptide segments
d) All of the mentioned
Explanation: A hydropathy plot is a method by which transmembrane segments of a protein can be identified if the amino acid sequence of the protein is known. Hydropathy plot assigns each site with a value to measure the hydrophobicity of an amino acid at a site and also at its neighbours. This helps to determine the arrangement of amino acid in the transmembrane domain, based on its hydrophobic character.
6. The channel in a membrane protein by which an ion or molecule can be transported in and out of the cell membrane is known as ___________
a) Permeation pathway
b) Permeate channel
c) Permeation channel
d) Channel pathway
Explanation: Transmembrane proteins act as channels from the intracellular and extracellular transport of ions and molecules. The channel through which the transport is done is called permeation pathway as it helps in the permeation of solutes and ions through the membrane.
7. Greater degree of unsaturation in fatty acids of cell membrane _________
a) Lowers the transition temperature
b) Raises the transition temperature
c) Has no effect on transition temperature
d) Doubles the transition temperature
Explanation: The lipid is converted from a liquid crystalline phase to a frozen crystalline phase. The temperature at which this change occurs is called transition temperature. The greater degree of unsaturation in fatty acids lower the transition temperature and it quickly achieves the crystalline frozen state due to high unsaturation.
8. GPI- anchored proteins show particular affinity towards ________
b) Hydrophobic tail
c) Lipid rafts
Explanation: Cholesterol and sphingolipids tend to self-assemble into microdomains highly ordered than surrounding lipid of cell membrane. Microdomains float within the fluid environment and are known as lipid rafts. Some proteins tend to become concentrated in the ordered microdomains and others remain outside the boundaries. GPI- anchored protein show particular fondness towards the microdomains.
9. FRAP is a technique to detect ___________
a) Membrane proteins
b) Transporting ions
c) Cholesterol patches
d) Cannot be determined
Explanation: Fluorescence recovery after photo bleaching (FRAP) is a process by which researchers follow the movement of proteins in cell membranes. A particular membrane protein can be labeled using a specific probe, such as a fluorescent antibody. If the labeled proteins are mobile, random movement of these molecules produce a gradual reappearance of fluorescence in an irradiated circle.
10. Antibody -coated gold particles are used in _______________
c) X-ray crystallography
d) Cannot be determined
Explanation: Single particle tracking or SPT is a method by which individual membrane protein molecules are labeled. Proteins are labeled with antibody-coated gold particles (approximately 40 nm in size). The movement of the labeled molecules is followed on computer-enhanced video microscopy.
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